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John Robison

John  Robison
John Robison is an expert on slot machines and how to play them. John is a slot and video poker columnist and has written for many of gaming's leading publications. Hear John on "The Good Times Radio Gaming Show," broadcast from Memphis on KXIQ 1180AM Friday afternoons. You can listen to archives of the show online anytime.

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Ask the Slot Expert: Are RNGs truly random?

11 Jan 2012

By John Robison

In your recent article about why the RNG creates large numbers, you reply included this: "First, the larger range means that the RNG can have a larger period -- that is, the number of numbers that can be generated before the RNG repeats itself."

If it is truly random the RNG could generate the same number many times before it reached 4 billion. So your answer may be accurate, but it leaves some confusion, to me anyway.

Thank you for your articles. I have learned much, but still trying to convince my wife.


Dear Chris,

Thanks for the kind words about my articles.

You hit the nail on the head with your supposition "if the RNG is truly random." The RNG in a slot machine is not truly random.

The RNG in a slot machine is really a P-RNG, a pseudo-random number generator. A P-RNG generates a stream of numbers that satisfy many of the requirements for randomness, but is not truly random because it is produced by a deterministic function.

A P-RNG function works by taking the last number (or maybe last two, three or more numbers) produced, performing some mathematical operations (like addition, multiplication, division) on it or them, and returning the result as the next number in the random number stream. There's nothing random about the mathematical operations (e.g., 2 plus 2 is always 4), so a given input will always yield the same output. The sequence will start repeating once we hit that given input again.

Keep in mind that this applies only to the raw output of the RNG. Once the program scales the RNG output to the number of virtual stops on a reel (using modulo arithmetic), it's possible to have the same virtual stop selected more than once in a row. The actual numbers from the RNG will have been different, though.

Best of luck in and out of the casinos,

Send your slot and video poker questions to John Robison, Slot Expert, at Because of the volume of mail I receive, I regret that I can't reply to every question.

This article is provided by the Frank Scoblete Network. Melissa A. Kaplan is the network's managing editor. If you would like to use this article on your website, please contact Casino City Press, the exclusive web syndication outlet for the Frank Scoblete Network. To contact Frank, please e-mail him at

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